Optical transmission may be used as a means for communication between separate integrated circuit die (also referred to as inter-die connections), and between components on the same die (also referred to as intra-die connections). Photonic devices are a class of devices that are capable of sourcing, controlling, and/or detecting optical transmission of signals.
The term “silicon photonics” relates to the study and application of photonic systems-that use silicon as an optical medium. Instead of, or in addition to, using silicon to facilitate the flow of electricity, silicon can be used to direct the flow of photons or light. Silicon is transparent to infrared light with wavelengths above about 1.1 micrometers. Silicon also has a high refractive index of about 3.5. The tight optical confinement provided by this high index allows for microscopic optical waveguides, which may have cross-sectional dimensions of only a few hundred nanometers, thus facilitating integration with current nano-scale semiconductor technologies that also employ silicon, such as complementary metal oxide semiconductor (CMOS) technologies. Silicon photonic devices can thus be made using existing semiconductor fabrication techniques. Further, because silicon is already used as a substrate for many electronic integrated circuits, it is possible to create hybrid devices employing both optical and electronic components integrated onto a single die.
In response to more demanding communication bandwidth, energy consumption, and performance standards for electronic devices such as semiconductor devices, photonic devices are increasingly being integrated with optical/electrical circuits to form a type of electronic-photonic device called an electronic-photonic integrated circuit. For example, in the semiconductor industry, photonic devices have various applications including communication within a die, between multiple die of a computer board, and between computer boards.
In inter-die communications via optical interconnects, each die on the circuit board can be interfaced with a photonic-electronic transmitter-receiver circuit, with two die operably connected via an optical waveguide. Likewise, in intra-die communications, optical waveguides may be used to connect components within a die, such as between an integrated optical source and a photonic detector.
FIG. 1 illustrates a block diagram of one example of a conventional photonic multiplexing system 100. The system 100 includes multiple carrier wave input devices 110a, 110b, 110c, 110d (generally referred to as an input device 110), each of which may be, for example, an optical source configured to generate an optical carrier wave at a respective transmission wavelength. For example, each input device 110 may include a coherent light source, such as a laser (e.g., a hybrid silicon laser or a gallium arsenide laser), or other appropriate light source known in the art.
The optical carrier wave from each input device 110a-d is transmitted to a respective resonant carrier wave modulator 120a-d along a respective optical waveguide 115a-d. Carrier wave modulators 120a-d are configured to receive respective optical carrier waves having different wavelengths from the respective input devices 110a-d, and to modulate data on the optical carrier wave that it receives. For example, carrier wave modulators 120a-d may be optical modulators configured to receive an optical data signal and output a modulated optical data signal, or electro-optical modulators configured to receive an electrical data signal from an electrically conductive interconnect and output a modulated optical data signal. The modulated light from each of the carrier wave modulators 120a-d is then combined and transmitted onto a single transmission channel (e.g., optical waveguide 140) using an optical multiplexer 130. The multiplexed light is transmitted along optical waveguide 140 to an endpoint (not shown) that may include, e.g., one or more photonic detectors for detecting optical transmissions, where the light is de-multiplexed and demodulated before being used by an endpoint device.
Wave guiding of an optical carrier wave through optical waveguides 115, 140 occurs through internal reflection of electromagnetic waves of an optical carrier wave at the interface between a higher refractive index inner core and a lower refractive index outer cladding. For example, the inner core of an optical waveguide 115 may be formed of a silicon (Si) or silicon-containing material, and may have a refractive index of approximately 3.5. The cladding of an optical waveguide 115 may be formed of a material having a lower index of refraction, for example, a SiO2 material with a refractive index of approximately 1.5.
Several components within a photonic system, and particularly components operating at a resonance frequency, can be affected by variations in temperature. Variations in temperature can result in changes in the device dimensions (due to thermal expansion) and refractive indices of materials. As one example, an optical laser providing one or more carrier wavelengths can be tuned by changing its temperature. As another example, changes in temperature can affect the operation of a resonant carrier wave modulator 120. The resonant frequency of a particular modulator 120 is controlled in part by the refractive indices of its resonant structures, which may change according to temperature, resulting in turn in a deviation of the resonant frequency of the modulator 120. Accordingly, certain photonic devices require a stable thermal environment to perform optimally.
One technique for providing a stable thermal environment for photonic devices includes active temperature control of one or more photonic devices, such as through an electric heating device. FIG. 2A shows a top-down view of photonic devices including an input device 110 (e.g., a laser), an optical waveguide 115, and a resonant carrier wave modulator 120, formed in a portion of a silicon die 230. A heating device 212 provides active temperature control of carrier wave modulator 120. Heating device 212 may be, for example, a resistive or inductive element, such as a polysilicon, silicon, or copper element, that is configured to receive energy (e.g., electrical energy) and output heat to the surrounding photonic devices. As shown in FIG. 2A, heating device 212 may be customized to modulator 120, by partially encircling modulator 120, or may have any other shape and be located near other temperature-sensitive photonic devices, as well as carrier wave modulator 120. In other embodiments, heating device 212 may be integrated with carrier wave modulator 120.
FIG. 2B shows a cross-sectional view of photonic devices formed in a portion of a silicon on insulator (SOI) integrated circuit die 230. Die 230 includes an input device 110 (e.g., a laser), an optical waveguide 115, and a resonant carrier wave modulator 120. Die 230 includes a substrate 232, which may be, for example, a bulk region of thermally conductive silicon. The SOI structure also includes an insulator region 233 (also referred to as a buried oxide or “BOX” region) composed of an insulating material, such as SiO2, which acts as a bottom cladding layer for an optical waveguide 115. Die 230 may also include an interlevel dielectric (ILD) region 236 having a lower layer composed of, for example, SiO2, formed above the device formation layer 235. The lower layer of ILD region 236 provides an upper cladding region for optical waveguide 115, and upper levels of the ILD region 236 are used for forming electrical connections in various locations of die 230, such as above the device formation region 235. Typically, photonic devices, including input device 110 (e.g., a laser), optical waveguide 115, and resonant carrier wave modulator 120, as well as heating device 212, are formed in the device formation region 235 above the substrate 232. Device formation region 235 may include regions of silicon for forming photonic devices, such as the inner core of optical waveguide 115 and modulator 120, and regions of a cladding material, such as SiO2, on the sides of optical waveguide 115 to serve as cladding surrounding a silicon waveguide core, and to serve as an insulating and mechanically supportive material for the devices formed in device formation region 235. Other photonic devices, such as other optical waveguides, lasers, filters, or photonic detectors, may also be formed in the device formation region 235, and may be subjected to active temperature control using a heating device 212.
Referring to FIG. 2B, while heating device 212 provides active temperature control of one or more photonic devices, such as modulator 120, in device formation region 235, it emits heat q that dissipates in all directions, including into the substrate 232, which is formed of thermally conductive silicon, and into the ILD region 236. This results in wasted heat flux and a less efficient heating device 212. Additionally, photonic devices in die 230 are typically thermally coupled to the substrate 232, and any global temperature variation in the die 230 may have an effect on the photonic devices.
Accordingly, it is desirable to thermally isolate heating devices and photonic devices in a photonic system, in order to improve efficiency and to provide more stable operation of the photonic devices.